Various type x-ray image converters utilizing photostimulable luminescent materials have long been known such as radiographic intensifier screens fluoroscopic screens, and x-ray image intensifier tubes. For example, in U.S. Pat. No. 3,617,743, assigned to the assignee of the present invention, there is disclosed lanthanum and gadolinium oxyhalide luminescent materials activated with terbium which exhibit superior conversion efficiency when employed to convert x-rays impinging on said phosphor medium to visible light. X-rays from a suitable x-ray source which pass through an object and impinge upon said phosphor medium form an immediate first light image which can be recorded on photographic film as well as produce a radiographic latent image which remains in said phosphor medium unless thereafter recalled by a suitable energy source to produce a second light image by thermoluminescent response. To further illustrate the nature of said latter phosphor behavior, there is disclosed in U.S. Pat. No. 3,996,472, also assigned to the present assignee, various rare earth oxyhalides co-activated with terbium and a second activator selected from zirconium and hafnium exhibiting superior thermoluminescent behavior in radiation dosimeters when subjected to heat stimulation. In more recently issued U.S. Pat. Nos. 4,346,295 and 4,356,398 there is disclosed laser means emitting light as a source of stimulating energy which produces the same type thermoluminescent response in various phosphor materials. A helium-neon laser in therein disclosed as the energy stimulation source for said purpose. Such a method of exciting the phosphor materials has been termed "photostimulation" wherein the energy from optical photons (ultraviolet, visible or infrared radiation) is said to stimulate the emptying of stored energy to produce light emission of the type termed "photostimulated luminescence" as distinct from "thermostimulated luminescence" which is produced by heating. Accordingly, the energy source of electromagnetic radiation hereinafter mentioned in describing the present invention as means for producing said photostimulated luminescent response in the phosphor medium herein being employed means ultraviolet, visible or infrared radiation.
X-ray image converters employing a conventional photostimulable phosphor medium still experience major problems with the optical image formed when retrieving the latent radiographic image from the phosphor medium. Said conventional phosphor medium is generally in the form of a porous layer of the phosphor particles bonded to a supporting substrate and with the void spaces between the individual phosphor particles causing some failure to retrieve all of the stored information as well as some loss of the emerging optical radiation. A partial solution to this problem is disclosed in U.S. Pat. No. 4,375,423, also assigned to the present assignee, wherein the phosphor particles are embedded or suspended in an optically transparent matrix which is selected or adjusted to have an index of refraction which is approximately equal to that of the phosphor embedded therein at the wavelength of light being emitted by the phosphor. This improvement enables more of the visible radiation being generated when the phosphor converts the impinging x-rays and forms an immediate first light image to escape from said medium. Spatial resolution for said immediate first light image is primarily determined by whatever spreading or scattering of the emitted light takes place in the phosphor medium. In accordance, with said improvement. phosphor particles of europium activated fluorochloride are embedded in a matrix of synthetic organic polymer such as the polymerization product of 2-vinyl naphthalene and vinyl toluene to provide superior optical coupling to photoelectrically responsive devices. There is further disclosed in said improvement various methods to manufacture such indexmatched phosphor medium for use in x-ray image converter devices suitable for various digital radiographic imaging equipment.
As distinct from said above described radiographic imaging systems wherein the entire visual image is displayed at one time, the present digital radiographic imaging system employs a moving beam of electromagnetic radiation to retrieve the latent radiographic image stored in said phosphor medium. Minimal spreading of said photostimulation energy as the moving energy beam penetrates the phosphor medium is most important to permit the latent image to be readout or recalled with higher resolution for a given thickness of said medium. Accordingly, resolution loss for the recalled visual image is experienced if the photostimulation energy becomes scattered within the phosphor medium beyond the specific lateral area being read at a particular time interval. Accurate retrieval of the stored information in the present system understandably further requires that all phosphor particles be accessed within the specific area being read at said time interval. This does not occur in the prior art x-ray screens due to excessive lateral light scattering which permits only shallow penetration of the phosphor medium by the photostimulation energy. Improved recovery of the latent radiographic image in the present manner thereby dictates minimal scattering of the photostimulation energy as distinct from that light or emission subsequently generated by the phosphor. Scattering of the visible or ultraviolet radiation produced when said photostimulation energy is selectively absorbed by the information containing phosphor particles is of lesser importance in the present type digital radiographic imaging system. The conventional photodetection means now employed in said type system are designed to collect all emerging radiation despite being scattered within the phosphor medium.
There also remains a need for larger size x-ray image converter constructions of this type. For example, digital radiographic systems are needed to scan relatively large areas of a patient such as employed for a chest diagnosis and which can exceed an area of one-half square foot or greater. To meet such large area requirement the index-matching matrix in which the phosphor particles are distributed must provide a self-supporting composite medium. To do so not only requires that the matrix constituent of said composite medium exhibit sufficent mechanical strength but also that the selected phosphor material be capable of forming a stable and uniform dispersion in said matrix constituent. It becomes especially important in the latter regard that a substantially void-free suspension of said phosphor particles result in order to preserve the desired optical improvement.
For still other reasons, it is required that a relatively large proportion of the index matched medium be occupied by the phosphor constituent. Absorption for most of the impinging radiation is desirable in the composite x-ray converter medium so that an accurate radiographic image therein is stored which dictates that sufficient phosphor be employed to achieve the desired degree of absorption. Accordingly, the phosphor and matrix constituents in the composite medium should be sufficiently compatible so that phosphor loadings can be achieved wherein the phosphor particles occupy a minimum weight fraction in said medium of at least 20-25%. Increasing the relative weight proportion of phosphor in the composite medium can also reduce absorption by the matrix constitutent of the photostimulation energy being employed to recall the latent radiographic image previously stored in said composite medium. Since it is further desirable in said retrieval steps to access all the embedded phosphor particles which may contain the x-ray stored information, any undue absorption of the electromagnetic energy producing such retrieval by the matrix constituent can effectively prevent an accurate retrieval.